Automatic volume control circuit



Jan. l8, 1938. F. H. DRAKE AUTOMATIC VOLUME CONTROL CIRCUIT Filled May 5, 1935 IIIIH llllll N i t Q. .zeghei .mmbfimq k w $5. N v NN.

A Rm 0 n T N w? R m m R A Patented Jan. 18, 1938 UNITED STATES PATENT OFFICE Frederick H. Drake, Boonton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 3, 1935, Serial No. 19,574

5 Claims.

My present invention relates to gain control for radio receiving systems, and more particularly to automatic gain control circuits for receivers.

It is one of the main objects of my invention to provide an automatic volume control arrangement for a receiver with low carrier level on the detector, the control system utilizing a diode rectifier provided by the cathode of ,an audio amplifier tube and a positive cold electrode thereof.

Another important object of the invention is to provide an automatic gain control system for a radio receiver of the type employing a carrier amplifier, a detector and an audio amplifier, the control system being characterized by the utilization of an output electrode and cathode of the audio amplifier as the electrodes of a diode rectifier, and the audio tube cathode bias resistor functioning to delay the gain control action.

Another object of this invention is to provide in a receiver an automatic gain control network which includes a carrier rectifier, the transmission circuit feeding the carrier rectifier being designed to suppress the flow of carrier energy to the rectifier over a predetermined portion of the receiver tuning range whereby automatic gain control action is secured over a desired portion of the tuning range, and is automatically eliminated over a different portion of the said range.

Another object of the present invention is to provide an audio amplifier tube network for a radio receiver, the audio tube having its plate and cathode arranged to rectify the received signal carrier component of the detector output circuit, the rectified carrier voltage being utilized for varying the gain of the networks ahead of the detector in a sense to maintain the carrier intensity level at the detector input substantially constant despite carrier level variations at the receiver input, and the audio amplifier having associated therewith balancing reactances for minimizing effects due to the existence of audio voltage between the plate and cathode of the audio amplifier,

Still other objects of the invention are to improve generally the simplicity and efiiciency of automatic volume control systems for radio receivers, and more especially to provide such control systems so that they operate reliably, and are readily constructed and assembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into efiect.

In the draWing:-

Fig. 1 diagrammatically represents a receiving circuit embodying the invention,

Fig. 2 graphically ilustrates the Ip-Ep characteristic of the audio amplifier in Fig. 1.

Fig. 3 shows a modification of the invention,

Fig. 4 is a circuit analysis of the embodiment shown in Fig. 3,

Fig. 5 graphically illustrates the operation of the invention as shown in Fig. 1.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, the signal receiving system shown in Fig. 1 is of a conventional type, and comprises a pair of tuned radio frequency amplifiers l and 2 arranged in cascade ahead of the detector tube 3. The detector is followed by an audio frequency amplifier tube 4, and the latter may be a tetrode of the screen grid type. If the receiver is of the tuned radio frequency amplifier type, then the tuning condensers 5, 6, and 1 may be uni-controlledly arranged for simultaneous tuning of their respective stages. If the reeciver is of the superheterodyne type, it will be obvious that the stages I and 2 may represent the intermediate frequency amplifiers preceding the second detector 3. In any case each of the amplifiers l and 2 follow conventional construction, and the circuits associated with tube show a conventional type of tuned radio frequency amplifier circuit.

The amplifier I includes the usual grid bias resistor 8 in its grounded cathode circuit, the bias resistor being shunted by the customary radio frequency by-pass condenser. The detector 3 is shown as being of the biased type, and includes a biasing resistor 9 in its cathode circuit, the resistor being shunted by a radio and audio frequency by-pass condenser. The plate circuit of the detector tube includes a source of positive direct current potential B, the latter being in series with a resistor l0 and the primary winding ll of transformer T.

The low alternating potential side of winding i! is connected to ground through a condenser I2, and a radio frequency by-pass condenser I3 is connected in shunt across the B voltage supply of the detector stage. The audio amplifier tube 4 has its plate connected to ground through a path which includes the secondary winding [4 of transformer T and the resistor I5. The grounded cathode circuit of tube 4 includes the usual grid bias resistor IE3 shunted by a by-pass condenser II. The signal input grid of tube 4 is connected to the junction of winding H and resistor it through a path which includes condenser IB and resistor It in series. The grid leak resistor 20 connects the grid side of condenser H! to the ground side of resistor l6.- s

A by-pass condenser 2! is connected between the screen grid electrode of audio amplifier 4 and the plate side of resistorlfi. The audio output circuit of tube 4 includes the primary winding of the audio transformer T1, the screen grid electrode of tube 6 being connected to the high alternating potential side of the transformer. The low alternating potential side of the primary winding of transformer T1 is connected by a lead 22 to the positive potential terminal of'source B. A b -pass condenser 23 is connected between the screen grid of tube 4 and ground. The secondary winding of transformer T1. may be connected to one, or more, additional stages of audio frequency amplification, and the output of the final audio amplifier may be connected to any desired type of reproducer.

The automatic volume control connection comprises the lead 25 which is connected to the signal input grid sides of the tuned input circuits of the radio frequency amplifiers of the receiving system. The lead 25 is designated by the letters AVC to designate that it is the automatic volume control path. A resistor-condenser network 26 is included in the AVG path, and functions to suppress the alternating current component of rectified carrier current. The AVC lead 25 is connected to the plate side of resistor l5.

The detected output energy flows through two paths to the network including tube 4. The audio frequency component of the detected output current is transmitted through coupling network l|9|8 to the control grid and cathode of i the audio amplifier, and the screen grid functions as the audio output electrode of the amplifier tube 4. On the other hand the transformer T functions as the transmission path for the carrier frequency component of the detected current. The latter current is impressed between the cathode and plate of tube 4, and the rectified carrier voltage appears across resistor l5. In other wordsthe cathode and plate of tube 4 cooperate to provide a diode rectifier, and. the rectified voltage appearing across resistor I is employed for varying the amplification of the controlled amplifiers and 2.

.It will be seen that since the grids of amplifiers l and 2 are connected to the plate side of resistor E5, the grids of the controlled amplifiers will become increasingly negative in bias as the carrier intensity level increases. The AVC action is delayed by virtue of the voltage developed across the cathode bias resistor [6. In other words, the bias resistor it not only functions to provide the normal operating bias for the signal input grid of audio amplifier tube 4, but it also provides a delayed negative bias, equal to the voltage developed across resistor 6, for the plate of tube 4 with respect to the cathode. When the carrier intensity peak voltage, impressed between the plate and cathode of tube 4, exceeds the value of the voltage developed across resistor I 6, then direct current voltage is developed across resistor l5,

and the grids of the controlled amplifiers l and 2 have their negative biases increased over and above their normal operating negative biases.

The detector tube 3 serves as a radio frequency amplifier, as well as a detector, in order to get a sufficiently high level for control on the diode rectifier. If the transformer T is properly designed, as by coupled tuned circuits, the AVG action may be obtained over a predetermined frequency band without having control action over the rest of the band. This is of advantage, for example, in pursuit work of the Navy and Army where communication with AVC is desired on one end of the tuning dial, and homing work without AVC is desired'on the other end of the dial. By proper design of the network T, shown enclosed in the dottedrectangle in Fig. 1, this transition of AVG action is automatically secured.

Fig.5 shows graphically two illustrative characteristicswhich can be secured-by the proper design of the network T. In the case of the full line curve F the AVG bias is shown produced over a substantial portion of the tuning dial adjustment, and eliminated over the remaining dial adjustment range. The dotted line curve D shows an. inverse charactertistic wherein AVC action is produced only over the latter portion of the dial range. Those skilled in the art will readily appreciate the constants and circuit elements necessary to secure these characteristics in choosing the design of the network T. For example, the network T may be a low pass filter network, and in such case the full line curve F will be secured. If the network T is a high pass filter then the dotted line curve D is secured. If desired the network T may be designed as a band pass filter network, and in such case the characteristic relating AVC' bias to tuning dial adjustment would have an intermediate portion of the tuning dial adjustment range result in AVC bias production.

The value of such an automatic switching arrangement resides in the fact that it is not necessary to employ any mechanical switches for eliminating the automatic volume control action. In homing work which covers from 200 to 1500 k, c., as when following a radio beacon, the AVG action is detrimental, and is desired to be eliminated. The network T automatically suppresses the transmission of carrier energy to the diode rectifier when the carrier energy has a frequency which is disposed. in that portion of the tuning range in which it is desired to suppress AVC action. Pursuit command operation usually covers a range of from 6200 to 7700 k. c., and AVC operation is desired in this range.

Those skilled in the art are fully aware of the manner of constructing a receiver. capable of covering frequency ranges of 200 to 1500 k. c.; and 6200 to 7700 k. 0. An all-wave receiver with plug-in coils to cover these ranges would be used. By winding transformer T to tune broadly through the pursuit command range of 6200 to 7700 k. c., or some part thereof, no AVC action will be obtained at other frequencies. The method of constructing transformer T for this purpose will depend on the frequencies to be covered. Generally speaking, the transformer will be wound to tune roughly with the output capacity of the detector and the plate capacity'of the audio tube; the primary and secondary windings being properly coupled. If the receiver is of the superheterodyne eype using different I. F. values fordifferent bands, then the transformer T will be designed 7 impress an audio potential on the plate.

to provide AVC action for all bands using a particular I. F. value, and no AVC on other bands.

In Fig. 2 is shown the I -E characteristic of audio amplifier tube 4. Assuming a screen grid voltage of 120 volts, the curve A designates the variation of plate current with plate voltage. For AVC action the positive plate current of the shaded area S is utilized. It will be noted that the positive plate current starts from zero plate volts. With the constants employed a plate current of 1 m. a. is secured with a peak plate voltage of 6 volts.

In Fig. 3 is shown an arrangement for balancing out the eifect of audio voltage which may exist between the plate and cathode of the audio amplifier 4; by virtue of condenser 2! the screen may The condenser E! is denoted by the symbol the resistor I5 is denoted by the symbol R1, and the bias resistor it is denoted by the symbol R2. A condenser C2 is connected between the cathode side of resistor R2 and the screen grid electrode of tube 4.

In Fig. 4 there is shown a circuit analysis of the balancing arrangement of Fig. 3. The balancing arrangement of Fig. 3 is shown re-arranged as a bridge arrangement, the resistors R1 and R2 being connected respectively between plate and ground, and cathode and ground. The two other arms of the bridge comprise the condenser C2 connected between cathode and screen, and the condenser 01 eing connected between screen and plate. An audio' voltage between screen and ground, represented as a diagonal of the bridge, will produce no effective voltage between cathode and plate if the following relationship exists:-

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A method of receiving signals of a predetermined frequency range which includes the steps of amplifying the signals at radio frequency, detecting the amplified signals, amplifying the audio component of detected signals, rectifying the carrier component of the detected signals, varying the signal amplification with direct current voltage derived from the rectified carrier component in a sense inverse to variations in the received signal carrier amplitude, and automatically suppressing the transmission of said carrier component for rectification during a predetermined portion of the said frequency range.

2. In combination, in a radio receiver, a signal amplifier provided with means for tuning it over a relatively wide range of signal frequencies, a detector, means, responsive to variations in received signal carrier amplitude, for automatically adjusting said amplifier gain in a manner to maintain the signal carrier amplitude at the detector input substantially uniform, said gain adjusting means having a resonant input circuit, and said last circuit being electrically constructed to transmit all signal carrier energy of frequencies lying within a predetermined band of said range and to reject signal carrier energy of frequencies lying within another band of the range whereby said adjusting means is automatically rendered inoperative when the tuning means is adjusted to tune the receiver over said second hand.

3. A method of receiving signals of a predetermined frequency range which includes the steps of amplifying the signals at radio frequency, detecting the amplified signals, amplifying the audio component of detected signals, rectifying the carrier component of the detected signals, varying the signal amplification with direct current voltage derived from the rectified carrier component in a sense inverse to variations in the received signal carrier amplitude, and automatically suppressing the transmission of said carrier component for rectification during a predetermined band of the said frequency range.

4. In a radio receiving system comprising a signal amplifier, a detector, an audio amplifier of the screen grid type, means for impressing upon the input electrodes of the audio amplifier the audio component of the detector output current, an audio output circuit connected to the screen grid of the audio amplifier, a diode rectifier circuit connected between the cathode and plate of the audio amplifier, means for transmitting to the diode circuit the carrier component of the detector output current, a gain control connection between the diode circuit and the signal amplifier, and means for balancing out audio voltages fiexisting between the cathode and plate of the audio amplifier.

5. In a radio receiving system comprising a signal amplifier, a detector, an audio amplifier of the screen grid type, means for impressing upon the input electrodes of the audio amplifier the audio component of the detector output current, an audio output circuit connected to the screen grid of the audio amplifier, a diode rectifier circuit connected between the cathode and plate of the audio amplifier, means for transmitting to the diode circuit the carrier component of the detector output current, a gain control connection between the diode circuit and the signal amplifier, and said transmitting means having a frequency transmission characteristic such that said carrier transmission tothe diode circuit is automatically prevented during at least one predetermined portion of the receiver tuning spectrum.

FREDERICK H. DRAKE. 

